The present invention relates to the art of generating magnetic fields, particularly, strong linear magnetic fields extending over relatively large distances. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with magnetic resonance spectroscopy, manufacturing materials with aligned dipoles, and the like.
Heretofore, most magnetic resonance imagers have generated the main or primary magnetic field axially through a plurality of annular magnetic coils. However, this air core design is relatively inefficient compared to an iron core magnet. Iron core magnets have well contained fringe fields, tend to be less costly to construct have improved patient access, have potential synergistic interaction between the pole faces and gradient coils, and have greater operational safety.
Others have heretofore constructed C or double-C-shaped iron core electromagnets for magnetic resonance applications. Even a relatively low field, e.g. 0.4 Tesla, magnet has a very large mass in the iron return path(s). In the prior art iron core magnets, the return path had not only a substantial cross section, but substantial overall length. In order to avoid distorting the magnetic field across the gap, the return path was positioned well away from the gap. That is, large C-shaped rather than shallow U-shaped return paths were utilized. The center point of the return path was disposed at a relatively large distance from the gap relative to the length of the gap in order to avoid distorting the magnetic field. The dual return paths of a double-C magnet tended to improve the symmetry or cancellation of distorting forces on the magnetic field.
As the field strength and gap distance increased, the cross section of the return path was also increased. The weight of the iron core increased approximately as the cube of the pole to pole spacing of the gap and linearly with the field strength. Due to this severe weight penalty, the gap of such magnets was minimized. To the extent such magnets were built for human imaging, the gaps were arranged vertically to accommodate the smallest front to back dimension of the human body.
The vertical gap iron core magnets had several drawbacks. First, the front to back dimension of the human body varied significantly from person to person. The gaps sized for the "average" person were too small and unusable for some persons. Further, when doing head scans, the patient was face to face with the upper pole creating a claustrophobic effect. The upper magnet also tended to interfere with range of motion studies, such as knee flexation studies.
The present invention contemplates a new and improved magnetic resonance apparatus and method which overcomes the above referenced problems and others.